Reports: ND650650-ND6: Chirped-Pulse Millimeter-Wave Spectroscopy: A Universal Probe of Photofragmentation and Pyrolysis Mechanisms via Species and Vibrational Population Distributions

Robert W. Field, Massachusetts Institute of Technology

PRF
Grant Report

We have demonstrated two major new applications of
chirped-pulse millimeter-wave (CPmmW) spectroscopy of exceptional relevance to basic
petroleum research.

A wealth of information about the identities and
relative abundances of reaction products, in particular the relative
populations of species, isomers, and vibrational levels, which we obtain by
CPmmW spectroscopy, reveals the reaction mechanism. Paradigm-changing results are
observed for a classical unimolecular photolysis reaction, which illustrate a systematically
accessible experimental path to the study of bimolecular pyrolysis reactions relevant
to biomass decomposition.

We
demonstrate isomer- and vibrational level-specific spectroscopy of the products
of 193 nm photolysis of vinyl cyanide.
This fundamental chemical reaction has attracted the attention of researchers
for decades. HCN was believed to be a dominant product
of 193 nm photolysis via a three-center transition state mechanism, where the CN
group captures the hydrogen from the adjacent carbon of the substituted
ethylene. In contrast, a four-center mechanism, where the CN group abstracts
the hydrogen atom of the other carbon atom of the ethylene, was believed to be
the dominant mechanism responsible for HNC products. In our investigation of
this photolysis reaction, we recorded CPmmW spectra of the photolysis products
of normal (CH2=CH-CN) and singly-deuterated (CH2=CD-CN)
vinyl cyanide. Surprisingly, the spectra in both cases showed almost equal yields
of HCN and approximately the same distribution of population among vibrational levels.
This observation is clearly incompatible with the commonly accepted three- vs.
four-center competition between the mechanisms of vinyl cyanide photolysis. We
find that when the hydrogen atom that would be expected to give HCN through the
three-center mechanism is replaced by deuterium there is negligible effect on
the HCN product yield. The DCN spectrum was observed as well, although it was of
significantly lower intensity than the HCN spectrum and had a different vibrational
population distribution.

The key advantages of CPmmW
spectroscopy, which include rapid acquisition of broad bandwidth spectra, are
demonstrated to be indispensible for fundamental chemistry research. The J=0-1 rotational
transitions of vibrationally excited HCN molecules, which are products of the
photolysis reaction, are spread in a vibrational pattern that covers several
GHz in the millimeter-wave spectral range. The ability to simultaneously record
these transitions enabled us to observee and distinguish the unique signatures
of the four-center and the three-center transition states along the vinyl
cyanide dissociation path. The slit jet enables systematic variation of the
amount of rotational cooling, by carefully positioning the photolysis laser with
respect to the collisional region of the supersonic expansion, were crucial in
these experiments.

Our
application of CPmmW spectroscopy to products of pyrolysis reactions is the
second major new direction initiated during the grant period. We have demonstrated the unsuspected importance of
bimolecular reactions with H-atoms in a Peter Chen type pyrolysis nozzle.
Previously unobserved reactions with H-atoms were observed and the reaction
yields were quantified.

The broad bandwidth of CPmmW spectroscopy facilitates
parallel detection of many pyrolysis reaction products and their vibrational
population distributions (VPD). We found, however, that, in contrast with
photolysis reaction experiments, the pyrolysis VPD contains almost no information
about the pyrolysis reaction transition state. The reason for this is the
collisional relaxation of both the rotational and vibrational populations
during the supersonic expansion from the Chen nozzle. For example, we found
that the VPD of the formaldehyde product of methyl nitrite pyrolysis

CH3ONO ^
[CH3O] + NO ^ CH2O + H + NO

is
very similar to the VPD of formaldehyde that was pre-mixed with the carrier gas
and ejected into vacuum through a heated pyrolysis nozzle.

The VPD of formaldehyde in both cases, however, was
found to be remarkable and counterintuitive. The common expectation is that
vibrational levels that lie higher than about 500 cm-1 above the
ground state are not significantly thermalized in the supersonic expansion. However,
in our CPmmW studies we found that the degree of relaxation is highly molecule-
and vibrational mode-specific. In formaldehyde, the out-of-plane bending mode
is highly populated and its corresponding vibrational temperature does not fall
below 50% of the temperature of the nozzle (1500 K). Other vibrational modes of
formaldehyde, however, relax to 10–20% of the nozzle temperature. The
non-Boltzmann VPD in formaldehyde can be attributed to the high symmetry of the
molecule, which imposes restrictions on transitions among quantum states during
the collisional relaxation, and the unique role played by Coriolis interactions
between some of the vibrational states.

Broadband detection of the pyrolysis reaction products
in a CPmmW experiment proved to be an information-rich path toward the discovery
of new chemical mechanisms at elevated temperatures. Pyrolysis of biomass is
commonly used to reduce the molecular weight of cellulose as an essential step
in its conversion into biofuels.

Our pyrolysis jet CPmmW results have revealed the
importance of chemistry mediated by hydrogen atoms. In collaboration with the research
group of Prof. G. B. Ellison we have investigated the thermal decomposition of
acetaldehyde. It is known to isomerize to vinyl alcohol, which unimolecularly decays
into ketene by eliminating H2. We have discovered that acetaldehyde
thermally decomposes to formaldehyde at very high (1800 K) temperature; however,
thermochemistry prohibits such a unimolecular reaction. In fact, it is the free
hydrogen atoms from dissociation of acetaldehyde that react with other
acetaldehyde molecules and produce the formaldehyde detected in our CPmmW
experiments. To investigate the H-atom addition reactions, we have added methyl
nitrite, which is a source of free H-atoms at moderate temperatures in the Chen
nozzle, to our acetaldehyde sample. This resulted in orders of magnitude larger
formaldehyde yields at significantly lower (1200 K) temperature. We conclude
that free H-atoms catalyze the cracking of acetaldehyde:

CH3CHO + H ^ CH2O + CH3

Isotopically substituted acetaldehyde was used to
distinguish its CH2O product from that produced in the pyrolysis of
methyl nitrite. The relative intensities of the precursor and product molecules
measured in the CPmmW spectrum make it possible, after some calibration, to
deduce the yields of different pyrolysis reactions. These initial observations
open a new direction of research in which pyrolytic reactions that involve
H-atoms can be discovered and quantified by CPmmW spectroscopy.